Color filter arrays can be provided by a number of different techniques. For example, dyes can be imbibed into deposited and patterned layers of mordant material. See, for example, U.S. Pat. No. 4,764,670 to Pace et al., and U.S. Pat. No. 4,876,167 to Snow et al., which describe such a process. This process suffers from several problems, such as the need to control the thickness of the mordant material layer and to precisely control the amount of dye used to achieve the appropriate color. An additional problem associated with this process is that the dye receiving layers swell upon the introduction of dyes, limiting the smallest dimension which can be attainable for use of very small filter elements. Another problem with this process is that dyes are susceptible to fading on exposure to light or heat.
Another method for making color filter arrays is by the transfer of dyes from a donor sheet into a receiving layer in a receiver. See, for example, commonly assigned U.S. Pat. No. 5,576,265 to DeBoer et al. which describes such a process. One problem with this process is that the dyes spread within the dye receiving layer in a receiver, resulting in loss in edge sharpness of the filter elements.
Alternatively, different layers of material with different indices of refraction can be deposited to form dichroic filters. Sce for example U.S. Pat. No. 5,246,803 to Hanrahan, et al., which describes such a process. One problem with the dichroic process is the complexity of forming multiple layers of different compositions and controlled thickness. Such a process requires precise control of a sputtering process in a high vacuum chamber, and suffers from the need to clean the vacuum chamber frequently to remove materials which build up on the interior surfaces and fixtures in the chamber.
An alternative to making color filter arrays from dyes or dichroic filters is to use organic pigments, which are more stable to light and heat than dyes, and are easier to process than dichroics, requiring no more than two or three layers. Organic pigments are thermally stable enough to be evaporated in a partial vacuum, and applied as a coating in what is known as physical vapor deposition (PVD), using a system such as that depicted in FIG. 1. A substrate 102 is positioned adjacent to an aperture mask 104. The aperture mask provides an aperture over a portion of the substrate. The material which is to provide the coating is placed into a source boat 100, which is heated by passing an electric current through it. Alternatively, the source boat can be heated by the application of radiant heating from a suitably placed heat source. Upon being heated under reduced pressure, the material vaporizes and travels from the source, impinging on mask 105. The portion of material vapor which passes through the opening in mask 105 travels along the lines 103, and between those lines, depositing on the substrate 102 and mask 104.
There are a number of problems associated with PVD using a conventional source boat. In certain cases, it is difficult to control the thickness and uniformity of the material deposited on the substrate, especially when it is desired to deposit more than one material simultaneously, such as may be the case in making a color filter array in order to fine tune the color produced. Such a co-deposition process typically requires simultaneous control of the evaporation rates from multiple source boats. Also, the use of appropriately placed sources or masks or moving substrate fixtures are typically required. In addition, the material may deposit on the masks and vacuum fixtures to such a degree that it flakes off, creating undesirable contamination and waste of the evaporant material, and requiring frequent clean-up. Further, the moving fixtures may generate undesirable particulate materials which may cause contamination of the substrate.
Some other problems with the PVD process as it is normally practiced are the need to use a large source-to-substrate spacing, which requires large vacuum chambers and large pumps to reach a sufficient vacuum, and the need for larger masks which cause low material utilization and build-up on the masks with the concomitant contamination problems. Very specific off-axis source location relative to the substrate, which is sometimes needed for uniform coating, also causes very poor material utilization. Still further, source replenishment problems exist for coating multiple substrates in one pump-down. In addition, when multiple layers are deposited, the process needs to be carefully monitored for the thickness of layers in the multiple colorant coatings in multiple cycles.